Valence tautomerism in a cobalt–dioxolene complex containing an imidazolic ancillary ligand

This work reports the synthesis, structural, spectroscopic and magnetic investigation of two complexes, [Co(bmimapy)(3,5-DTBCat)]PF6·H2O (1) and [Co(bmimapy)(TCCat)]PF6·H2O (2), where bmimapy is an imidazolic tetradentate ancillary ligand and 3,5-DTBCat and TCCat are the 3,5-di-tert-butyl-catecholate and tetrachlorocatecholate anions, respectively. Their structures have been elucidated using single crystal X-ray diffraction, showing a pseudo-octahedral cobalt ion bound to a chelating dioxolene ligand and the ancillary bmimapy ligand in a folded conformation. Magnetometry displayed an entropy-driven, incomplete, Valence Tautomeric (VT) process for 1 in the 300–380 K temperature range, while 2 displayed a temperature independent, diamagnetic low-spin cobalt(iii)–catecholate charge distribution. This behaviour was interpreted on the basis of the cyclic voltammetric analysis, allowing the estimation of the free energy difference associated with the VT interconversion of +8 and +96 kJ mol−1 for 1 and 2, respectively. A DFT analysis of this free energy difference highlighted the ability of the methyl-imidazole pendant arm of bmimapy favouring the onset of the VT phenomenon. This work introduces the imidazolic bmimapy ligand to the scientific community working in the field of valence tautomerism, increasing the library of ancillary ligands to prepare temperature switchable molecular magnetic materials.

To monitor the reference pseudo-electrode, the redox pair ferrocenium/ferrocene (Fc + /Fc) was used as reference standard. Potential values are expressed in relation to the Fc + /Fc pair. The working potential range used was -1800 to +1800 mV and the sweep speed 100 mV s -1 .

Magnetometry.
Samples employed for magnetic measurements consisted of pressed microcrystalline powders of 1 and 2, wrapped in Teflon(TM) tape. The magnetic characterization was performed on Quantum Design MPMS (Magnetic Properties Measurement System) equipment provided with a 5 T magnet. The magnetization (M) dependence with the absolute temperature was investigated between 380 and 60 K using a magnetic field (B) of 10 kOe, and between 60 and 10 K with a field of 1 kOe to prevent magnetic saturation. After subtraction of the diamagnetic contribution of the sample holder and the sample, evaluated with Pascal's constants, 12 magnetic susceptibility per mole (χ M ) was evaluated as χ M = M M /B.

Thermogravimetric Analysis.
The thermogravimetric curve was measured with a Shimadzu DTG-60 Analyzer, applying a 10 K min -1 scan rate, under a 50 mL min -1 flow rate of argon.

X-ray diffraction studies
Single crystal X-ray diffraction data for both complexes was collected on a Bruker D8 Venture diffractometer using Mo K  radiation ( = 0.71073 Å). Data collection, data reduction and cell refinement were performed by using the Bruker Instrument Service v4.2.2 and SAINT V8.34A softwares. 2 The structure was solved by direct methods using the SHELXS program, and refinement was performed using SHELXL based on F 2 through full-matrix least squares routine 3 within winGX package. 4 Empirical multi-scan absorption corrections using equivalent reflections were performed with SADABS program. 5 All non-hydrogen atoms were refined with anisotropic displacement parameters.
Hydrogen atoms were set in calculated positions and refined using the riding model. 6 The structures were drawn using the Mercury software. 7 Compound 1 show instability losing its diffracting properties if the crystal is kept out of mother liquor at ambient conditions. This occurs presumably due to partial or total loss of the lattice solvents. For this reason, low completeness data was obtained. Lattice water molecule, tetrachlorocatechol ligand and PF 6anion are disordered in 2. The first two are disordered over two with positions. Atoms occupancies were freely refined. DFIX restraint was applied to P-F bond length in PF 6anion. SADI command was applied in order to have similar bond lengths for C-C and C-O in the tetrachlorocatechol ligand. Anisotropic displacement parameters of some atoms of catechol were restrained to be the same and phenyl ring was restrained to be flat.

DFT Calculations
The computational studies of the complexes were carried out with DFT level theory, using TPSSh functional, def2-TZVP basis function and D3 dispersion interaction energy correction in ORCA 5.0.3 quantum package. 9-11 Every geometry of each structure was optimized to an energy minimum in PES, both for the singlet ls-Co 3+ Cat state and for the quintet hs-Co 2+ SQ state of each complex.          Potentials are reported in volts vs the Fc + /Fc redox couple. E pc , E ac and E 1/2 report, respectively, the cathodic, anodic and half-wave potentials. ox-red is the difference of the half-wave potentials of the metal-and ligand-centred processes. 8 Peak-to-peak separations (E) and oxred are reported in mV.

Metal-based process
Ligand-based process